Hitherto, methacrylic acid is industrially produced by a method comprising subjecting a raw material, for example, methacrolein to a vapor phase catalytic oxidation reaction using molecular oxygen. This method uses a catalyst comprising a heteropolyacid compound containing phosphorus and molybdenum. The yield of methacrylic acid in such a method is largely influenced by the performances of the catalyst used (e.g., a conversion and a selectivity). Therefore, the production methods of the catalyst comprising a heteropolyacid compound are variously studied in order to improve the performances of the catalyst.
For example, JP-A-05-177141 discloses 1) a method for producing a heteropolyacid catalyst containing phosphorus, molybdenum and cesium comprising the steps of drying an aqueous slurry containing phosphorus, molybdenum and cesium to form a solid heteropolyacid compound, suspending the resulting solid heteropolyacid compound in water to form a suspension, and then adding an aqueous solution of starting compounds for the catalyst containing phosphorus and molybdenum but no cesium to the suspension, followed by drying and calcining, and JP-A-2002 233758 discloses 2) a method for producing a catalyst for the synthesis of methacrylic acid comprising a heteropolyacid compound containing phosphorus, molybdenum and at least one element X selected from the group consisting of potassium, rubidium, cesium and thallium, which method comprises the steps of drying an aqueous slurry containing molybdenum, phosphorus and the element X to form a solid heteropolyacid compound having an atomic ratio of the element X to molybdenum of 2.5:12 to 12:12, and adding the heteropolyacid compound to an aqueous slurry which is separately prepared so as to have an atomic ratio of the element X to molybdenum of 0.05:12 to 0.4:12, followed by drying and calcining.
However, the catalysts for the preparation of methacrylic acid produced by the method 1) or 2) described above are not necessarily satisfactory in conversion and selectivity.